Molded resistors



Filed Feb. 7, 1952 IN V EN TOR. MEL V/N 7"/ERMAN Y H/s A T Top/vers MOLDED REsIsToRs Melvin Tierman, North Adams, Mass., Electric Company, North Adams, of Massachusetts assignor to Sprague Mass., a corporation The present invention relates to new and improved high temperature resistance elements and to methods of making the same.

Much effort has been expended in the past to produce composition resistors which would be capable of use over a wide temperature range, and which would possess highly desirable temperature coeicients of resistance and resistivity. None of these attempts has been completely successful. This is particularly true in the production of resistance elements which can be used at ambient temperatures of from 200 C. to about 225 C. for extended periods.

It is an object of the present invention to produce resistors which can be used under such elevated temperature conditions. Another object of the invention is to produce molded resistance compositions which can be employed over an extremely wide temperature range, and which possess desirable temperature coetcients ot resistance. These and further objects of the invention, as well as the advantages of it, will be apparent from the following detailed description and the appended claims, reference being made to the single ligure of the accompanying drawing showing in partial cross-section a resistor exemplifying this invention.

The above aims of the invention are broadly achieved by producing a composition resistor employing a polytetrahaloethylene resin binder with selected filler materials by an improved process. Terminal leads may be applied to the unit produced by these steps by means of a special silver-particle-containing silicone conducting paint, by molding etc, The entire unit is preferably positioned within a polytetrahaloethylene impregnated braided glass ber sleeve which serves to effectively insula'te and protect the unit. Also the resistor may be coated with a layer of resin not containing conducting particles.

The preferred resin used with this invention is a polytetrauoroethylene compound sold under the trade name Teon, although other polytetrahaloethylene resins such as polytriuorochloroethylene, polydiuoro dichloro ethylene and polytrichlorouoro ethylene are also satisfactorily employed. Polyvinyl uoride may also be used, although the maximum operational temperature will be somewhat lower than when a polytetrahaloethylene is employed. Whenever the term Teon is used in this specification it is to be understood that the other polyhaloethylene resins indicated may be used in its place.

ln the actual production of the resistance unit a colloidal suspension of all of the ller, and conducting ingredients may be prepared with the aid of a small pro portion of a dispersing agent. This suspension may then be added and mixed with a Tetlon suspension or a similar variety. The colloidal mixture may then be coagulated by the use of heat or an organic solvent which will break an emulsion, or both, in order to produce a pasty mass from which is ltered the excess liquid. This mass is then pre-shaped in an extrusion press, dried at a rela` tively low temperature to vaporize the readily volatile ICC constituents present, and then dried at an elevated temperature below the sintering point of the resin until the remaining volatile material is driven ott, and cooled. The dried resistance material is next placed and contined in a mold and sintered at a temperature above the transition point of the resin, but below the decomposition point of the resin.

After the unit produced in the above manner is sintered, a silicone paint may be applied to its ends, dried, and appropriate leads are attached, as by soldering or capping. It is also possible to use the modified types of conventional terminal connections with this variety of resistance element, as for example, silver end caps may be applied or silver plated copper wire may be molded into the body of the resistor before sintering. A unit with leads applied may be used in any type of retaining protective casing, or in certain applications it may be employed without any such casing. However, it is preferable to slip it into a fiber glass sheath which has been impregnated with a resin, such as a polytetrahaloethylene resin, in order to protect it during handling and use. Also a coating of Teflon without conductive ller may be applied.

l have discovered that the temperature coetiicient of resistance of resistors employing Teflon binders may be made very low by a specific combination of the Teilon binder and the conducting particles with an inert filler. According to my discovery, l have found that optimum results are obtained when the ratio of the total amount of conducting particle plus inert filler to the resin binder is from at least 1:3 to about 2:1. (With higher ratios the physical stability and electrical characteristics are inferior.) When larger relative amounts of resin are employed the temperature coefficient becomes very high and non-linear irrespective of the nature of the conducting material. Since in most instances the actual content of conducting material such as graphite or carbon lies in the range of perhaps 4% to about 25% it is necessary according to my invention that an inert filler be added in order to present the composition within the ratio limits given above. Suitable inert fillers are mica, talc, china clay, ground glass, titanium dioxide, Whiting and aluminum oxide. All the ratios cited herein are determined on a weight basis.

As indicated above the conducting particles are ordinarily present in amounts from about 4% to a maximum of about although higher values may be used on occasion for exceptionally low resistance values or other special characteristics. For the lower resistance values graphite, powdered metals such as silver, aluminum, lead, and the like are preferred. For the higher resistance ranges carbon black produced by any one of the numerous processes is particularly suitable due to its availability in tine particle size and its low cost. However, many other conducting and even semi-conducting llers may be employed such as N and P type ceramic, alloy, and elemental semi-conductors and the like. In all cases, for maximum .stability and optimum temperature coefficient characteristics, the linear coeeient of expansion of the gross non-resinous portion of the composition should be from about 3)(10-6 to about 20 l0r- Values appreciably higher than this will result in larger temperature coeiiicients of resistance even when the composition falls within the ratio limits noted above.

The colloidal suspension of Teon, or other resin, used in this invention may be produced in a similar manner to the method used in producing the rst suspension. l't is preferred to use within from about to 50 grams of .solid with about 80 mls. of water. Frequently it is desirable to purchase such a colloidal suspension from one of the commercial producers of the product. ln general, these resin suspensions contain from about 40 to about solids with about 'from 0.05% to about 'f' l fof "aedetergent `dispersin g agent.

are preferably smaller than; 325 mesh.

.Tho two colloidal .suspensions whoorrolacod are .adde'dto roue 'another in the orogori -oroponoowo .es A""to.;o.bf'f1i11 a' resistonoeunl :of the. desired oompostoo- "Preferably the two' Suspensions are iodslsd- While boing subjected *to agitation 'bye .oolilmor1l beatotorjprooollor tvpjol stirring dovioo. 3 Tho oombinod .suspensions once produced may be coagulate'd in any'ofthestandard methods kuowoto tho/art tooroduooavot'pasw Sludge. One .method whichcan-be used involvesmerely. heating :the jmijiture until suiicientwa'ter is vaporizedtowtugealo the suspension. second irnethodfis; to add. acetone, ethyl aloohol... or another Similar. organic., Solvent which Will break the sus'pension .j 'lt is-.preferred however, to comy bine bothof these means together and toV heat themixture. a t frorn 40?. C. @190i C. while adding from 5% to 25%aotoneor-etliyla1ooho1 t fljhasludge resulting fromV mistreatment lterer] or hoth'ef1'vv i. 'se vseparatedjfrlom, lthe bulk of tbeliluidj ac .COIding ,to J,conventiofnzf1-lf practice. .Thefpasty mass resulting fis thenV pre-shaped byeonventional. 2itrusion equipment lintofthe final shape desiredfor the jresistance element. It 4is preferred4 to. use a common-plunger type device similar to a well-known lard type' pressin order to :accom- .plishnthis.. Pressures. of from 5- to 100 ljbs. per square ...inchmay .belused. atthoi1ah it. isi. Preferred to.. employ betweenilO and-f 5 Dlbs; per square incli pressure,V depending upon the degree of dryness of the resistance composition. f 7; 5.,.

The we t masses. which have been pre-shaped are dried "Qa `romabout.roomntemperature to.,.ab out;.105 .fC. for fr m4'tof 8 hours afterthey have beerrpre-sh'iapedin order to remove thelast vestige of water p'res'ent:. Tem- ...fper'aturesfof-fromlxaboutSOf' C. toaboutV l25l. .C.ware '.{svuitable forjthisirst dryiingstagej. Thenthe elements f. a" rheat ed totemperaturesin excess of 200 C; but below .thekguring point otfpthe resin (3201@ forfTeon) for ...perodsof from ..15 minutoslogehours fia-ofdr.: to-,tlrive foi any. and all volatilipzible material present.,y The .units faregdried by this Ystage procedure sogtbatthevwill, not '.','be eomefriable. If carefinhandling is exercised they may befdried directlyat an elevated temperature... i After they have beenJl-cooled thetormedpelletsare l p Vaeed in molds, andi are lsubjectedto. pressures( pf from 5,000 to 50,000 lbs. per lfsqllarein ch. fonperiodsof. 3 :pto 5 minutes. at normalworking,temperaturesofA from ...2W-.Culo aboutBO? C. Thisstep servesrto compaotthe individual particles-.withimthe unit insuch arnanne r that The solids used duetive unitis 'producedL While lowerpressuresgthan are ...Y indicated above are possible, witlmsuch. lower. pressures it isnot .possible to conveniently. develop `the high strength `l`of :the nal unit during .the/ tcuring-L- step Thiscuring Y stepgis carried out by heating theresistors above. thecur- ...ina temperature of the. resin .and blow; hodeompfts tionpoint ofthe resin for periods of from one-half hour .to .1 hour,.depending, upon the-sizeof the actualnnit. With .-.Teon, temperaturesof -iron 1 327"- C to 3903.. C. are '.lused. After the heating the units are brought back to room temperature` HLeefhengthe ends of3theelements Ito..whichterminal conan vorganic solvent;A such asv Xylene, benzenefhexane, ytoluene, or thelike,-forming'u the balance ofthecomposition. A minimum o'f"l0%` solvent should beused in tlielpaint with most silicones. lAmong '-threisilicones which can be usedare the aryl-silieones, the alkyl silioones Asuitable for the formation of heating-setting resins. Included .among the same are ethyl, methyLw-propyl, lbutyl, avmyl, phenyl, xylyl, naphthyl silicones, alone or co-condensed l withone another. Halogen substituted-silieones-may..also be used. Further details on suitable silicone resins may be found in An lntrodution to the Chemistry of Silicones by Rochow, second edition 1951, published `by 5 lohn Wiley & Sons, New York, New York. These silicones may be -dened asrgano substituted polysiloxanes in which the organo groups are linked by carbon to silicon bonds.

.T ho. silicone-Point .maysltotooploo -to.-.th .o.nd otto@ unit either by.drongusoiayioaor brushing. audits: cured by heating at temperatures of from 250-tC.-..to.325." C. for .Periods @from Smiautesfo? houts- -Qfdirtary Silver paint such asV has 'been ui'sdin th'prio ant 'is Lnot suit able for use with .the.resistors ..,f.this invention, because it peels off these resistors .at about 200 C. or lower.

When the terminal paint has been cured various end leads may be attached.v IThe preferred methodis to v pressure forfcefn oictagoiitalA` cap'wliiehhas aiterminal lleadnextendingfrom itfn to "each 'terminal painted'end 2t)A of the resistor. High temperature soldedrs y'such :1s-'copperf, 'zine` or oopperZinc-'silver'may' be` adapted'i'to' attach suitltleltd'lls- The finished unitisfV thenf preferably 'slippedI within: a braided glassfslee've which has been dipped a lf'ljeilon"resinggsuspension ofmthey Vtype*ernployedin form- 7^ ing the resistornitselfand finally sintered atfrromabout 327 C. -toabout *3190? C. 'fora period ofr lS'jminutes to 2 hours. Obviously other resins 'besidesvTeon may be employed. vAlso other *protective` coverings sch` as are commonlvemployed -the'art maynbev utilized. .Thogfollowingspoio esempio of. fhofplfoss Plot/Led.. in amidon@ Wth'ftho' invention is sivo o o ly UY'for purposesfof'illu'stra mn. and is` not to beA cbnsidrd las limiting theinvehtive in any way. 35

i `Exmitjile 45 minutes,. 'lfhejsuspensionlwasthen added'rto 30 mls. of',Y [a Teflonsuspens-iop` '(Drrr Plont5 manufactured by. Loo. PontdeNomoors &.Q.o ,..W. mogto'. 1.2.613-

ware) containing approXirnatelySS%' water-and ef'small amount of.. D11 .Pont .Dupon'ol Stabilizor- .,Theaotilal msmiaingtottthe two .ingredients was .done by. addioe. the list-.Suspension tothe .second in. a vessel oquipood...with propeller stirrer. While stirring was carried on contents were heated-at.-40 C. for one-half hour, `then 60 mls. of

.- acetone Wa'sadded and the mixture `stirred .forfanother 5 minutes. The resultant products were filtered on a common-laboratory Buchner funnel using suctionproduced by Van"uasr'air'at'pr. The'pasty mass' obtainedbyA this Hprocedure extrudedthrougli a 1A diameter ,'orice A'underafpressure of'appto lartelyWS A smiwwas dried 65. lioonofsilvorffiiant. *Ufo Tellen suspension'of'the`type"'used"in the frna oii of the resistor, and curing this casing at 375 C. for 45 minutes.

Using the same general procedure indicated in the above example, numerous resistors of varying compositions have 3. A high temperature resistor possessing an extremely low temperature coecient of resistance consisting essentially of a dense mass of sintered particles of polytetratluoroethylene resin in which are uniformly dispersed parbeen made. For convenience a summary of these units, 5 ticles of carbon, graphite and an inorganic, electrically together with certain of their properties, are given in the insulating filler, the approximate proportions of said mateappended table. rials being 40 parts of resin, 3.5 parts of carbon, 3.5 parts Weight Percent (dr)7 basis) Weight Re i t 't 5,2m 'gem Teflon Ratio olgxrrfxir?, Resitaxnce Peieeent Cgl'afg Whiting other 111mb" of@ ohms per o 43. 4 5. s 1.31 1, 530 12, 200 0.070 38. 5 5. 9 1.59 2, 480 11, 40o 0. 064 32s 10.0 2. 05 445 3, 720 0. 057 35.0 10.0 1.75 102 733 0.055 38.5 5.9 55.5 1.50 s45 0, 309 0.040 35. 2 11. s 52. 0 1. 75 30. 0 209 0. 051 39.0 5.0 55.0 1.55 27,000 102,000 0.050 39.3 4.0 55.7 1.55 00,000 3,350,000 0.070 35.2 10.0 53.8 1. 75 9.9 85.9 0.071 34. 4 7. 0 55. 0 1. 75 112 990 0. 052 35 4 5. 0 58.5 1. 75 1, 050 15, 900 0. 054 40. 0 5. 0 55. 0 1. 50 435 4, 190 0. 070 35.2 52.0 11.5 gmph1te... 1. 75 344 3,440 -0. 010 32.8 47.2 20.0 graphite... 2.05 s 159 0. 015 47.0 53.0 graphite.-. 1.13 .25 1.79 0. 055 54.0 35.0 graphite... .55 1.50 15.0 0.055 72.0 1s.0grapn1te .25 5,150 34,900 -0. 159 75.0 25.0 graphite-.. .33 5.4 35.7 0.047 72.0 28.0 graphlte. .39 4.1 35.9 0.029 35.4 3.5 55.5 3.5 aquadag 1.75 8.7 83.7 -0. 013 30.4 3.5 55.5 3.5 aquadag 1.75 10.5 92.7 -0. 013 40.0 3.5 53.0 3.5 aqudag 1.50 3.3 31.5 0. 017 40.0 3.5 53.0 3.5 aquadag.. 1. 50 3.3 32.5 -.0. 014

In the accompanying drawing the figure shows the above of graphite and 53 parts of inorganic filler, and connection type of resistor 10. Its molded core is indicated at 12 terminals electrically bonded to spaced portions of the with the terminal paint 14 at each end. The end caps 16 mass. are fitted over these ends and have their terminal leads 18 4. The resistor of claim 2 wherein the terminal connecextending therefrom. Around the core there is positioned tions are bonded in place by a surface coating of a cured the braided glass fiber sheath 20. silicone resin containing silver powder in an amount of As many apparently widely diterent embodiments of at least twice the amount of the silicone by weight. the invention may be made without departing from the 5. A- high temperature resistor comprising a substanspirit and scope hereof, it is to be understood that the tially non-porous body consisting essentially of sintered invention is not limited to the specic embodiments hereof, particles of polytetrafluoroethylene resin and electrically except as delned in the appended claims. conducting and electrically non-conducting particles, the I claim: weight ratio of the total of said particles to the resin being l. A high temperature resistor consisting essentially of from about 1:3 to about 2:1, the linear coecient of a sintered uniformly dispersed mixture of a polyhaloethexpansion of the non-resinous portion of the composition ylene resin containing at least one luorine atom per being from about SXlO- to 20 l06 per degree cenethylene unit, with electrically conducting particles and tigrade, and the exposed resistor surface is encapsulated electrically insulating particles, the weight ratio of elecwith a sheath consisting of braided glass liber impregnated trically conducting and electrically non-conducting parwith polytetrafluoroethylene resin. ticles to the resin being from about 1:3 to about 2:1, and t terminal connections electrically connected to spaced por- References Cited 111th@ le 0f this Patent tions of the sintered mixture. UNITED STATES PATENTS 2. A high temperature resistor having a substantially non-porous body consisting essentially of sintered parenlman "get" 3 1933 ticles of polytetrauoroethylene resin and uniformly ru 0V' l 1938 dispersed electrically conducting and electrically non- 23 32116 Schmld Oct' 19 1943 conducting particles, the weight ratio of the total of said 4001009 Bnfbaker et al May 14 1946 particles to the resin being from about 1:3 to about 2:1, 2,419,655 Reiser APR 29, 1947 the linear coecient of expansion of the non-resinous 2,472,801 Bal'eld Ct al June 14, 1949 portion of the composition being from about 3 X 10-6 to 2,512,280 Lemmers June 20, 1950 20x10-s per degree centigrade, and connected terminals 60 2,559,077 Johnson et al July 3, 1951 electrically bonded to spaced portions of said body. 2,609,470 Quinn Sept. 2, 1952 

1. A HIGH TEMPERATURE RESISTOR CONSISTING ESSENTIALLY OF A SINTERED UNIFORMLY DISPERSED MIXTURE OF A POLYHALOETHYLENE RESIN CONTAINING AT LEAST ONE FLUORINE ATOM PER ETHYLENE UNIT, WITH ELECTRICALLY CONDUCTING PARTICLES AND ELECTRICALLY INSULATING PARTICLES, THE WEIGHT RATIO OF ELECTRICALLY CONDUCTING AND ELECTRICALLY NON-CONDUCTING PARTICLES TO THE RESIN BEING FROM ABOUT 1:3 TO ABOUT 2:1, AND TERMINAL CONNECTIONS ELECTRICALLY CONNECTED TO SPACED PORTIONS OF THE SINTERED MIXTURE. 